The invention relates to failed fuel detection and more particularly a apparatus for detecting and locating failures of the fuel cans of liquid-cooled nuclear reactors, for example nuclear reactors cooled by an upward flow of molten sodium.
Since fuel failures allow fission products to escape into the coolant, they must be detected and located as quickly as possible. In liquid-cooled reactors having no pressure tubes, such determination of the location of fuel failures is complicated by the mixing of the liquid coming from the various fuel sub-assemblies at their outlets. One solution for this kind of reactor is to take at the outlet of each fuel sub-assembly a small portion of the liquid flow which has passed therethrough, mix the sample with a gas having a large neutron capture cross-section, separate the gas and monitor it. Since the gas becomes radioactive in contact with the contaminated liquid, a sub-assembly having a failed fuel is shown up by the activity of the gas coming from the corresponding sample. A single mechanicaly sampling pump might be used (or if the liquid is conductive an electro-magnetic pump) with a selector valve which connects the inlet of the pump to the outlets of all the sub-assemblies in the core in succession. This method has serious drawbacks, since it is difficult to design and manufacture a liquid sample selector valve which correctly operates in the conditions to which it is subjected for long periods of time.
Moreover, if the pump is mechanical its moving parts are fragile.